ASK THE HORT AGENT
Question Will charcoal ashes from my BBQ grill hurt my plants?
Answer Doesn’t it sound silly to say, “you can eat food cooked only inches over charcoal, but don’t dare spread those ashes on your centipede lawn”? If it’s suitable to burn directly under your food, then it’s probably not that harmful.
Charcoal is residue consisting of carbon obtained by removing water and other volatile constituents from wood. It is usually produced by heating wood in the absence of oxygen. Charcoal is often used to filter water or adsorb odors. The porosity of activated charcoal gives it the ability to filter. Ironically, it is the odor or “flavor” of burning charcoal briquettes that makes it desirable as a heat source.
The formation of charcoal briquettes is one place where potentially harmful products may be added. Briquettes are made from a combination of charcoal (heat source), brown coal (heat source), mineral carbon (heat source), borax (press release agent), sodium nitrate (ignition aid), limestone (uniform visual ashing), starch (binder), raw sawdust (ignition aid) and possibly additives like paraffin or lighter fluid to aid in lighting. "Natural" briquettes are made solely from charcoal and a starch binder. On the other hand, fire eliminates most of these substances.
Charcoal ashes remain when the organic (carbon) material has oxidized (burned off). These ashes are primarily minerals, especially potassium (K). This mineral is the third number on a bag of fertilizer (N-P-K, 10-10-10). Potassium (also called potash) is a salt. Like nitrogen, too much potassium can cause fertilizer “burn” symptoms in plants. In excess, potassium can also limit the availability of other nutrients.
However, it is not the potassium that causes people to fear the potential toxicity of charcoal briquette ash. It is the additives that cause the charcoal to either bind together or burn quicker. The most harmful additive is usually added by the cook – lighter fluid. Good barbecue chiefs let the lighter fluid “burn off” before cooking. Here is a simple rule to follow. If it smells like lighter fluid and it tastes like lighter fluid, then it probably has lighter fluid in it. Petroleum-based lighter fluid is not good for plants (or people). Let the lighter fluid burn off before grilling.
Ashes from coal contain sulfur dioxides. This is probably not good for your plants, but who grills steaks over a coal fire?
For more info about wood ash, visit http://www.hort.purdue.edu/ext/woodash.html
If you do not have access to the internet, then call the Extension Office at 893-7533, write us at PO Box 1089, Lillington, NC 27546, or email me at firstname.lastname@example.org
One of the most important historical applications of wood charcoal is as an ingredient of gunpowder. Typically gunpowder is not toxic by itself. It is the addition of lead that causes acute toxicity upon impact.
Gary L. Pierce
Horticulture Extension Agent